If I get normal eirections can I takw viagra to get bigger?

Use of Viagra is to treat ED. By your own admission, you aren't having that difficulty. Therefore, you are not likely to get a prescription for an ED med.

Seeking it to try to enlarge or harden your erection is not wise, as there are some side effects and consequences.

If you want a better experience, try not having sex as often, and your erections are likely to be harder and more responsive to stimulation. Alternatively, you and your gf could go shopping at a sex positive shop for a toy or dildo for her to play with or just to try out.